With the development of various electronic devices, there are also many power supplies developed to provide reliable power for electronic devices. Refer to FIG. 1 for a conventional power supply. The power supply has a switch unit 5, a rectifier unit 1, and a power factor regulation unit 2. The rear side of the power factor regulation unit 2 is coupled to a primary power system and a regular power system. When the switch unit 5 is turned on, an input power will enter the power supply and pass through the rectifier unit 1 and the power factor regulation unit 2. The power factor regulation unit 2 modulates the current phase of the input power and provides the power for the primary power system and the regular power system. The primary power system includes: a primary pulse modulation unit 31, a transformer unit 32 and a rectification/output unit 33. The turn-on/turn-off of the primary pulse modulation unit 31 is controlled by the regular power system. The regular power system includes: a voltage-detection unit 41, a regular power output unit 42, a power administration unit 43 and a far-end start switch 6. When a user triggers the far-end start switch 6, the power administration unit 43 generates a far-end start signal (Vcc) to drive the primary pulse modulation unit 31 to make the primary power system output power. In the conventional technology, the switch unit 5 must be turned on before the power supply obtains the input power. The switch unit 5 may be a single throw switch or a double throw switch. With the persistently rising output of a power supply, the switch unit 5 has to withstand higher and higher current. Thus, the switch unit will have a larger volume and a higher cost. Further, with the rising current, the switch unit 5 may generate sparks during switching. Under an environment of high humidity, or with a wet hand, a user touching the switch unit 5 may get an electric shock. Therefore, the switch unit 5 should be improved to solve the abovementioned problems.